I. Field
The present invention relates generally to transformers and more particularly to an off-circuit tap changer used to adjust with a rolling, spring-biased moving contact the output voltage of a transformer by changing its tap winding when the transformer is de-energized.
II. Background
FIG. 1 is a view of a conventional off-circuit tap changer system 10. The system 10 includes in general a drive mechanism 18 coupled to an off-circuit tap changer device 20. A transformer tank 5 is coupled to the system 10 between the drive mechanism 18 and the off-circuit tap changer device 20. The drive mechanism 18 includes a handle 16.
An off-circuit tap changer device 20 is used to adjust the output voltage of a transformer by changing its tap winding when the transformer is de-energized. For a three-phase transformer, the conventional tap changer device 20 is a three-phase switch, with six (6) stationary contacts 25 in each phase (5 operating positions) evenly configured over 180°. There is one moving contact 30 for each phase installed on a main shaft assembly 40.
FIG. 2 is a cross-sectional view of a conventional tap changer device 20 and FIG. 3 is an end view schematic of the device 20 along the plane 3-3 of FIG. 2. The off-circuit tap changer device 20 includes a plurality of circulating circuits 22A, 22B and 22C, each of which is comprised of one moving contact 30 and an array of stationary contacts 25. As a frame of reference, in the view of FIG. 2, the moving contact 30 is in contact with a pair of adjacent stationary contacts 25A and 25B. Through driving, by the rotation of the main shaft assembly 40, the moving contact 30 rotates with the main shaft assembly 40 to an angle to mate with another pair of stationary contacts in the array of stationary contacts 25, thus finishing a tap changing operation. Structure characteristics of the moving contact 30 is of a sector structure.
The motion of the moving contact 30 of the conventional off-circuit tap changer device 20, during tap changing, is a sliding motion. This construction causes more wear between the curved surface of the moving contact 30 and the contacting surfaces of the two adjacent stationary contacts 25; and leads to improper contact condition. The array of stationary contacts 25 is supported by parallel insulating panels 35.
Another disadvantage of the conventional off-circuit tap changer device 20 is the main shaft assembly 40. The main shaft assembly 40 includes a plurality of insulating pipe sections 42A, 42B and 42C. In this example, there are three insulating pipe sections. The construction of the main shaft assembly 40 with the plurality of insulating pipe sections 42A, 42B and 42C increases the machining job load and installation tolerance of the tap changer device 20. Furthermore, the installation or assemblage of these pipe sections 42A, 42B and 42C to construct the main shaft assembly 40 is generally complicated.
Each of the plurality of circulating circuits 22A, 22B and 22C is integrated with a metal connector 45 which is inline between adjacent insulating pipe sections. The alternating pipe section and connector configuration creates the main shaft assembly 40 to be rotated.
Thus, there is a need for techniques for adjusting the output voltage of a transformer with minimum surface wearing between the moving contact and the stationary contact.